The present invention relates to the field of integrated circuits, and in particular to a bias circuit for obtaining accurate on-chip time constants and conductances, and a variable resistor circuit for use with a bias circuit.
Generally, it is difficult to realize precise and consistent on-chip impedances levels in an integrated circuit. On-chip impedances are typically realized by transistors biased in the triode region, or by on-chip resistors. When a MOS transistor is used to approximate a resistor, variations in drain-source conductances makes impedance levels difficult to predict. Drain-source conductance can vary as a result of a number of different factors, with the temperature dependence of carrier mobility being a major factor. The resistivity of on-chip resistors can vary as a result of a number of different factors, including manufacturing process fluctuations which affect carrier concentrations, and also the temperature dependence of carrier mobility. The impedance of "identical" on-chip resistors can easily vary by 30% from wafer to wafer at room temperature, and can vary by over 100% with temperature changes.
The difficulty in predicting impedance levels in integrated circuits affects, among other things, the predictability of the gain of an integrated circuit as such gain is often determined by the ratio of transistor transconductance to the conductance of on-chip resistors.
In many integrated circuits which make use of MOS technology, the speeds realized by the circuits are dependent on the ratio g.sub.m /C (where g.sub.m represents the proportionality between the drain current and gate-source voltage of a MOSFET transistor in the active or saturation region and C=the value of an on-chip capacitor). However, as mentioned above, drain-source conductance can vary with temperature fluctuations, thus making on-chip time constants difficult to accurately predict control. The unpredictability of time constants makes it difficult to accurately predict the speed of an integrated circuit as the speed of the circuitry is directly proportional to the time constants which are realized by the circuit. The unpredictability of time constants also makes it difficult to accurately predict the frequency response of integrated filters as such response is dependent on the time constants.
There are a number of bias circuit designs which attempt to provide predictable and stable transistor transconductances. For example, J. M. Steininger, "Understanding Wide-Band MOS Transistors," IEEE Circuits and Devices, Vol.6, No.3, pp.26-31, May 1990, discloses a constant-g.sub.m bias circuit in which transistor transconductances are matched to the conductance of a resistor. Examples of constant-g.sub.m bias circuits can also be seen in J. Ryan et al., "A Magnetic Field Sensitive Amplifier with Temperature Compensation," ISSCC Digest of Technical Papers, pp. 124-125, February 1992; and D. Johns and K. Martin, "Analog Integrated Circuit Design", Wiley, 1997. The disclosures of the above three references are hereby incorporated by reference.
The basic principle of operation of the constant-g.sub.m bias circuits disclosed in the above references is that such circuits make the gate-source voltage difference of two different sized transistors having the same (or proportional) drain currents equal to the voltage across a resistor also having the same or a proportional current. This results in a transistor transconductance that is proportional to the inverse of the resistor value with the proportionality factor being highly predictable and process and temperature independent. However, as mentioned above, precise on-chip resistances are difficult to achieve, with the result that the absolute value of transistor transconductance can not be precisely pre-determined and controlled when an on-chip resistance is used.
It is therefore desirable to provide a circuit which allows accurate on-chip time constants and conductances to be precisely obtained and controlled independent of temperature, power supply, and process variations. In this respect, a circuit having an on-chip impedance solution and transistor transconductances that can be precisely predicted and controlled substantially independent of temperature, power supply and process variations is required. It is also desirable to provide a low noise variable resistor device which can be used as an on-chip impedance solution with a loop circuit for achieving a broad range of resistances.